The way you show in your diagram using a 2 line splitter with one leg going to the modem and the other going to your existing 4 line splitter for your TVs is how I would do it. Modems generally seem to be more sensitive to line levels than set top boxes, so unless your connection is very hot, the lower loss from using a single two line splitter in front of the modem would likely produce the best results. If you are planning to do a self-install, just try it and look at the line levels in the modem to make sure that they are within specs.--
We can never have enough of nature.
We need to witness our own limits transgressed, and some life pasturing freely where we never wander.